Infrared-Active Phonon Modes in Single-Crystal Thorium Dioxide and Uranium Dioxide

Authors

Sean Knight, University of Nebraska-LincolnFollow
Rafal Korlacki, University of Nebraska-LincolnFollow
Christina Dugan, Air Force Institute of Technology
James C. Petrosky, Air Force Institute of Technology
Alyssa Lynn Mock, University of Nebraska - LincolnFollow
Peter A. Dowben, Univesity of Nebraska - LincolnFollow
J. Matthew Mann, Air Force Research Laboratory
Martin M. Kimani, Air Force Research Laboratory
Mathias Schubert, University of Nebraska-LincolnFollow

Date of this Version

3-2020

Citation

Journal of Applied Physics 127, 125103 (2020)

DOI: 10.1063/1.5143724

Comments

Copyright 2020 by AIP Publishing

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 127, 125103 (2020) and may be found at https://doi.org/10.1063/1.5143724

Abstract

The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO₂) and uranium dioxide (UO₂), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO₂ and UO₂ are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. At room temperature, our results for the mode pair’s resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO₂, our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon–phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational mode pair is detected within the reststrahlen band for both ThO₂ and UO₂. Elevated temperature measurements were performed for ThO₂ in order to study the mechanisms by which the phonon parameters evolve with increased heat. The observed change in the TO resonant frequency is in excellent agreement with previous density functional calculations, which only consider volume expansion of the crystal lattice. This suggests that the temperature-dependent change in the TO frequency is primarily due to volume expansion. The change in the main mode pair’s broadening parameters is nearly linear within the temperature range of this study, which indicates the intrinsic anharmonic scattering (via cubic anharmonicities) as the main decay mechanism.